The report also includes statistics on terrorist activity worldwide, and said 11,774 terrorist attacks in 92 countries occurred in 2015.
The department's acting coordinator for counterterrorism, Justin Siberell, briefed reporters Thursday on the report's content, saying the document was used to assess the effectiveness of the effort to combat terrorism and to determine where to best place resources.
On Iran, the report said that country "remained the foremost state sponsor of terrorism in 2015, providing a range of support, including financial, training, and equipment, to groups around the world."
The report also said that Iran was continuing to provide arms and cash to terrorist groups like Hezbollah and Iraqi Shia terrorist groups, including Kata'ib Hizballah (KH). Both groups are designated Foreign Terrorist Organizations in the report.
Iran is one of three listed state sponsors of terrorism, the others being Syria and Sudan. Cuba was removed from the list last year.
Siberell added that the department was "concerned about a wide range of Iranian activities to destabilize the region."
Iran's designation and continued sponsoring of terrorism is bound to fuel criticism of the Iran nuclear deal. Critics of the deal have charged that the removal of economic sanctions would allow Iran to increase its support of terror groups.
During an interview in Davos, Switzerland, Secretary of State John Kerry admitted in January
that some of the money made available to Iran by the removal of sanctions would "end up in the hands of the IRGC or other entities, some of which are labeled terrorists," referring to Iran's Revolutionary Guard Corps.
The report also described the global terrorist threat in 2015 as "increasingly decentralized and diffuse," noting that ISIS once again was "the greatest threat globally."
It included a statistical annex prepared by the University of Maryland, which said that worldwide there were 11,774 terrorist attacks in 92 countries in 2015 which resulted in more than 28,300 total deaths.
The report noted that the total number of terrorist attacks in 2015 decreased by 13% when compared to 2014.
Siberell said, "This represents the first decline in total terrorist attacks and resulting fatalities worldwide since 2012."
However, he added that terrorist activity had increased in Afghanistan, Bangladesh, Egypt, Syria and Turkey in 2015.
While the State Department lists the Middle East and North Africa as the "primary theater for terrorist activity," it also highlights the terrorist threat posed to Europe "from foreign terrorist fighters who returned home to Europe to plot and carry out attacks."
The report says that in 2015, ISIS and al Qaeda "increased their focus on staging mass-casualty attacks," noting attacks in Paris, Lebanon and Turkey.
The State Department recently issued a travel alert
to U.S. citizens considering traveling to Europe, warning about the risk of potential terrorist attacks throughout the continent ahead of the busy summer travel season.
The report notes that Sub-Saharan Africa also "experienced significant levels of terrorist activity" from groups like Boko Haram in West Africa and Somalia-based al-Shabaab in east Africa.
Overall, the report lists 13 "terrorist safe havens" around the world where "terrorists are able to organize, plan, raise funds, communicate, recruit, train, transit and operate." These safehavens include remote areas in Southeast Asia, the Middle East and South America.
The report lists 58 "Foreign Terrorist Organizations," including ISIS, al Qaeda and various affiliates and branches of those two organizations.